Low-yellowing aqueous clear powder coating dispersions, method of making the dispersions, and process for producing clearcoat finishes with the dispersions

ABSTRACT

The present invention relates to a process for producing single-coat or multicoat clearcoat finishes on primed or unprimed substrates by applying at least one powder coating dispersion, in particular a powder clearcoat slurry, to the substrate and then curing the resulting film(s) thermally and/or with actinic radiation, in which at least one polyvalent inorganic acid and/or oxalic acid are or is added to at least one powder coating dispersion, in particular to a powder clearcoat slurry.

This application is a National Phase Application of Patent ApplicationPCT/EP00/03276 filed on Apr. 12, 2000.

FIELD OF THE INVENTION

The present invention relates to a process for preventing the yellowingof single-coat or multicoat paint systems produced using powder coatingdispersions, especially powder clearcoat slurries.

BACKGROUND OF THE INVENTION

Powder clearcoat materials and also aqueous powder clearcoat dispersionsare part of a trend for automobile coatings and, owing to theirenvironmental compatibility, are displacing those materials which havebeen used to date in finishing, namely solvent borne one-componentclearcoat materials based on melamine resins and also the likewisesolvent-based two-component clearcoat materials comprising polyacrylateresins with polyisocyanates as crosslinkers. Aqueous powder clearcoatdispersions, also referred to by those in the art as powder clearcoatslurries, have the advantage over the powder clearcoat materials thatthey can be processed using conventional liquid paint technology.Furthermore, they can be applied at low film thickness and hence theycontribute to the sparing use of resources. Processes of this kind areprior art. For example, the U.S. Pat. No. 4,268,542 describes a processin which a multicoat system comprising two or more coats is applied to asubstrate. The surface of the substrate is first of all coated fully orpartly with a powder coating material. Thereafter a powder coatingdispersion, consisting of synthetic resin particles dispersed in anaqueous medium, is applied and the substrate thus treated is dried in asingle step by the action of heat.

DE 196 13 547 discloses an aqueous powder clearcoat dispersion which,through the use epoxy-carboxy systems, permits crosslinking attemperatures below 160° C. However, this system exhibits a relativelystrong tendency to yellow in comparison to the conventional clearcoatmaterials based on melamine resin or on polyisocyanate.

The German patent applications 197 41 555.5 and 197 44 561.6 describe apowder clearcoat material in which a glycidyl-containing additionpolymer is crosslinked with tris(alkoxycarbonylamino)triazine andpolycarboxylic acids and/or carboxy-functional polyesters. This powderclearcoat material may likewise be formulated as an aqueous powderclearcoat material, which in relation to the prior art has advantages inrespect of etchability by environmental chemicals, and also exhibitsimprovements in terms of yellowing. Particularly with the clearcoatingof white shades, however, the yellowing resistance is still critical andrequires particular care at the production stage. When selecting the rawmaterials and intermediates that are used for this purpose it isnecessary to ensure particularly high purity and very low intrinsicyellowing, which is a disadvantage. Especially when preparing powdercoating dispersions in the production plant, in batch sizes which exceedthe laboratory scale, such batches may occasionally be found to beparticularly critical in terms of yellowing, following application to awhite undercoat.

It was therefore an object of the present invention to provide a processfor producing single-coat or multicoat paint systems using powderclearcoat slurries that no longer has the disadvantages of the prior artbut instead provides paint systems which in particular no longer showany yellowing, without detracting from the other, positive qualities ofthese paint systems.

BRIEF SUMMARY OF THE INVENTION

What has been found, accordingly, is a process for producing single-coator multicoat clearcoat finishes on primed or unprimed substrates byapplying at least one powder coating dispersion, in particular a powderclearcoat slurry, to the substrate and then curing the resulting film(s)thermally and/or with actinic radiation, in which at least onepolyvalent inorganic acid and/or oxalic acid are or is added to at leastone powder coating dispersion, in particular to a powder clearcoatslurry.

DETAILED DESCRIPTION OF THE INVENTION

In the text below, the novel process is referred to as the process ofthe invention.

The present invention additionally relates to powder clearcoat slurriesto which at least one polyvalent acid and/or oxalic acid acid have orhas been added and also to the single-coat or multicoat clearcoatfinishes, and coated articles, produced by applying these powder coatingdispersions, particularly powder clearcoat slurries.

In one variant of the process of the invention, the polyvalent acidsand/or the oxalic acid are also added to the aqueous basecoatdispersion, leading to a further suppression of yellowing.

The novel powder coating dispersions or powder clearcoat slurries arereferred to below as powder clearcoat slurries of the invention and thenovel single-coat or multicoat powder clearcoat finishes are referred toas coatings of the invention.

In the light of the prior art it was surprising, and unforeseeable forthe skilled worker, that as a result of the process of the invention thepowder clearcoat slurries of the invention and the coatings of theinvention would no longer have the disadvantages of the prior art, whilesuffering no impairment of their other, positive qualities.

Furthermore, it is an advantage of the present invention that theabsence of color-falsifying yellow hues in the coating of the inventionresults in better reproducibility of the shade of the color-impartingcoat.

The essential constituent of the powder clearcoat slurry of theinvention is at least one polyvalent inorganic acid and/or oxalic acid.

As polyvalent acids it is possible to use, with preference, phosphoricacid and its derivatives, sulfuric acid and its derivatives, and boricacid, with particular preference phosphoric acid and its derivatives,with very particular preference phosphoric acid. Instead of or inaddition to these acids it is possible to use oxalic acid.

In the context of the process of the invention, the acids are added tothe powder clearcoat slurries of the invention in effective amounts. By“effective amounts” here are meant amounts which on the one handreliably prevent yellowing without destabilizing the powder clearcoatslurry and/or harming the performance application profile of the paintsystems. The skilled worker is therefore able to determine theappropriate amounts by means of simple preliminary tests.

Of particular advantage are amounts from 0.1 to 2000 ppm, preferablyfrom 1 to 1500 ppm, with particular preference from 100 to 1200 ppm, andin particular from 800 to 1100 ppm, based in each case on the overallamount of the powder clearcoat slurry of the invention. They aretherefore employed with preference.

In accordance with the invention, the acids are added to the startingproducts or to the intermediates at any step in the process of preparingthe powder clearcoat slurry of the invention, or to the finished powderclearcoat slurry of the invention, the last-mentioned variant being ofadvantage and therefore being employed with preference.

To produce the coatings of the invention, the powder clearcoat slurry ofthe invention is applied in accordance with the process of the inventionto the primed or unprimed substrate to be coated. Viewed in terms of itsmethod, the application has no special features, but instead takes placewith the aid of the customary and known methods such as spraying, knifecoating, brushing, dipping, flow coating or rolling, but especiallyspraying. The process of the invention starts from the substrate to becoated. The substrate may be of metal, wood, plastic, glass, ceramic orpaper or may comprise these materials in the form of composites.Preferably, the substrates in question comprise automobile body parts orindustrial components, including containers, made of metal, orfurniture.

The substrate may have been provided with a primer. In the case ofplastics, the primer in question is what is known as a hydroprimer,which is cured prior to the application of the coating material of theinvention, particularly of the aqueous basecoat material or of theclearcoat material. In the case of metals, especially automobile bodyparts, the primer comprises customary and known, cured electrocoats towhich a primer-surfacer is applied and baked.

Accordingly, the powder clearcoat slurry of the invention and theprocess of the invention is also suitable for applications outside ofautomotive finishing, particularly for the coating of furniture and forindustrial coating, including coil coating and container coating. Thepowder clearcoat slurry of the invention is especially suitable as acoating over basecoats, preferably in the automobile industry. It isparticularly suitable as a single-coat or multicoat clearcoat finishover aqueous basecoats based on polyesters, polyurethane resins, andamino resins, as are described in the patents EP-B-0 089 497 or EP-A-0522 420.

Advantageously, the powder clearcoat slurry of the invention and theprocess of the invention will be effected as part of the wet-on-wettechnique employed in connection with the OEM finishing of automobiles,which is a further advantage of the powder clearcoat slurry of theinvention.

Application by the wet-on-wet technique embraces the followingworksteps:

(I) applying a basecoat film, particularly an aqueous basecoat film, toa primed or unprimed substrate,

(II) flashing off the resulting basecoat film and subjecting it tointerim drying,

(III) applying a clearcoat film to the basecoat film, particularlyaqueous basecoat film, and

(IV) curing the two wet films thermally and also, where appropriate,with actinic light.

The powder clearcoat slurry of the invention used in the process of theinvention is curable thermally and/or with actinic radiation.

In the context of the present invention, the term “thermal curing”denotes the heat-initiated curing of a film of a coating material, forwhich normally a separate crosslinking agent is employed. This iscustomarily referred to by those in the art as external crosslinking.Where the crosslinking agents are already incorporated in the binders,the term self-crosslinking is also used. In accordance with theinvention, external crosslinking is of advantage and is thereforeemployed with preference.

In the context of the present invention, actinic radiation meanselectron beams or, preferably, UV radiation. Curing by UV radiation isnormally initiated by free-radical or cationic photoinitiators.

Where thermal curing and curing with actinic light are employed togetherfor one coating material, the term “dual cure” is also used. For thecure it is possible to employ the customary and known methods such asheating in a forced air oven or with IR lamps and also, whereappropriate, irradiation with UV lamps.

The powder clearcoat slurry of the invention may have the compositiondescribed in the patents DE 196 13 547, DE 197 41 555.5 and DE 197 44561.1, except that in addition it comprises the acids for use inaccordance with the invention.

Particularly advantageous powder clearcoat slurries of the inventioncomprise:

a) at least one epoxide-containing binder containing, based on thebinder, from 0.5 to 40% by weight of copolymerized glycidyl-containingmonomers, and

b) at least one tris(alkoxycarbonylamino)triazine and at least onepolycarboxylic acid, in particular a straight-chain dicarboxylic acidand/or a carboxy-functional polyester as crosslinking agent

or alternatively

a) at least one tris(alkoxycarbonylamino)triazine and at least oneoligomeric or polymeric, epoxide-containing crosslinking agentcontaining, based on the crosslinking agent, from 0.5 to 40% by weightof copolymerized glycidyl-containing monomers, and/or a low molecularmass, epoxide-containing crosslinking agent, and

b) at least one carboxyl-containing polymer as binder

and for both variants

c) at least one of the above-described polyvalent inorganic acids oroxalic acid, and

d) where appropriate, at least one polyol,

and/or

e) where appropriate, constituents which permit curing with actiniclight.

The composition of the powder slurry of the invention may vary widelyand may be tailored to the particular end use. In accordance with theinvention it is of advantage if the powder clearcoat slurry of theinvention, based on the respective solids, comprises the constituentsa), b) and c) and also, where appropriate, d) and/or e) in the followingamounts:

a) from 40 to 85, preferably from 50 to 80, with particular preferencefrom 55 to 75% by weight,

b) from 10 to 30, preferably from 12 to 25, with particular preferencefrom 15 to 23% by weight,

c) from 0.1 to 2000 ppm, preferably from 1 to 1500, with particularpreference from 100 to 1200, in particular from 800 to 1100 ppm, andalso

d) from 0 to 30% by weight, and

e) from 0 to 40% by weight.

Suitable epoxy-functional binders a) for the powder clearcoat slurriesof the invention are, for example, polyacrylate resins containingepoxide groups, which are preparable by copolymerizing at least oneethylenically unsaturated monomer containing at least one epoxide groupin the molecule with at least one further ethylenically unsaturatedmonomer that contains no epoxide group in the molecule, at least one ofthe monomers being an ester of acrylic acid or methacrylic acid.Polyacrylate resins of this kind containing epoxide groups are known,for example, from the patents EP-A-0 299 420, DE-B-22 14 650, DE-B-27 49576, U.S. Pat. No. 4,091,048 or U.S. Pat. No. 3,781,379.

Examples of suitable monomers which contain no epoxide group in themolecule are alkyl esters of acrylic and methacrylic acid, especiallymethyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, n-butyl acrylate, n-butyl methacrylate, secondary-butylacrylate, secondary-butyl methacrylate, tert-butyl acrylate, tert-butylmethacrylate, neopentyl acrylate, neopentyl methacrylate, 2-ethylhexylacrylate or 2-ethylhexyl methacrylate; amides of acrylic acid andmethacrylic acid, especially acrylamide and methacrylamide;vinylaromatic compounds, especially styrene, methylstyrene orvinyltoluene; the nitriles of acrylic acid and methacrylic acid; vinyland vinylidene halides, especially vinyl chloride or vinylidenefluoride; vinyl esters, especially vinyl acetate and vinyl propionate;vinyl ethers, especially n-butyl vinyl ether; or hydroxyl-containingmonomers, especially hydroxyethyl acrylate, hydroxyethyl methacrylate,hydroxypropyl acrylate, hydroxypropyl methacrylate, 4-hydroxybutylacrylate or 4-hydroxybutyl methacrylate.

Examples of suitable epoxy-functional monomers are glycidyl acrylate,glycidyl methacrylate or allyl glycidyl ether.

The polyacrylate resin containing epoxide groups normally has an epoxideequivalent weight of from 400 to 2500, preferably from 420 to 700, anumber-average molecular weight Mn (determined by gel permeationchromatography using a polystyrene standard) of from 2000 to 20000,preferably from 3000 to 10000, and a glass transition temperature Tg offrom 30 to 80, preferably from 40 to 70, with particular preference from40 to 60, and in particular from 48 to 52° C. (measured with the aid ofdifferential scanning calorimetry (DSC)).

The preparation of the polyacrylate resin containing epoxide groups hasno special features but instead takes place in accordance with thecustomary and known polymerization methods.

A further essential constituent of the powder clearcoat slurry of theinvention is the crosslinking agent b).

Advantageously, at least two different crosslinking agents b₁) or b₂)are employed.

Of these, the first crosslinking agent, b₁), comprisestris(alkoxycarbonylamino)triazines and their derivatives. Examples ofsuitable tris(alkoxycarbonylamino)triazines are described in the patentsU.S. Pat Nos. 4,939,213, 5,084,541 or EP-A-0 624 577. In particular, thetris(methoxy-, tris(butoxy- and/ortris(2-ethylhexoxycarbonylamino)triazines are used.

Preference is given to the methyl butyl mixed esters, the butyl2-ethylhexyl mixed esters, and the butyl esters. They have the advantageover the straight methyl ester of better solubility in polymer melts.

The tris(alkoxycarbonylamino)triazines and their derivatives may also beused in a mixture with conventional crosslinking agents. Particularlysuitable for this purpose are polyisocyanates which are different fromthe tris(alkoxycarbonylamino)triazines. It is also possible to use aminoresins, examples being melamine resins. In this context it is possibleto use any amino resin suitable for transparent topcoats or clearcoats,or a mixture of such amino resins.

Resins of this kind are well known to the skilled worker and are offeredby numerous companies as commercial products.

In the case of the second crosslinking agent, b₂), the compounds inquestion comprise carboxylic acids, particularly saturated,straight-chain, aliphatic dicarboxylic acids having from 3 to 20 carbonatoms in the molecule. Instead of or in addition to them it is alsopossible to use carboxy-functional polyesters. With very particularpreference, decane-1,10-dicarboxylic acid (dodecanedioic acid) is used.

In order to modify the properties of the powder clearcoat slurries ofthe invention, it is possible as well to use other carboxyl-containingcrosslinking agents, in minor amounts. Examples of suitable additionalcrosslinking agents of this kind are saturated branched or unsaturatedstraight-chain di- and polycarboxylic acids and also thecarboxyl-functional polymers that are described in detail below asbinders b).

In a second variant, the powder clearcoat slurries of the invention maycomprise an epoxy-functional crosslinking agent a) and acarboxyl-containing binder b).

Examples of suitable carboxyl-containing binders b) are, for example,polyacrylate resins which are prepared by copolymerizing at least oneethylenically unsaturated monomer containing at least one acid group inthe molecule with at least one further ethylenically unsaturated monomerthat contains no acid groups in the molecule.

Examples of suitable oligomeric and polymeric, epoxy-functionalcrosslinking agents a) are the above-described binders a) containingepoxide groups.

Examples of suitable low molecular mass, epoxy-functional crosslinkingagents a) are low molecular mass compounds containing at least twoglycidyl groups, especially pentaerythritol tetraglycidyl ether ortriglycidyl isocyanurate.

The binder a) containing epoxide groups and the carboxyl-containingcrosslinking agent b) of the first inventive variant, and, respectively,the carboxyl-containing binder b) and the epoxy-functional crosslinkingagent a) of the second inventive variant, are used generally in aproportion such that there are from 0.5 to 1.5, preferably from 0.75 to1.25, equivalents of carboxyl groups per equivalent of epoxide groups.The amount of carboxyl groups present may readily be determined bytitration with an alcoholic KOH solution.

The epoxy-functional binder a) or the oligomeric or polymericepoxy-functional crosslinking agent a) contains, in copolymerized form,vinylaromatic compounds such as styrene. In order to limit the risk ofcracking on weathering, however, the amount is not more than 35% byweight, based on the binder a) or on the crosslinking agent a).Preferably, from 10 to 25% by weight are copolymerized.

The powder clearcoat slurry of the invention may comprise polyols d).

Suitable polyols d) include all low molecular mass compounds, oligomers,and polymers which contain at least two, preferably at least three,primary and/or secondary hydroxyl groups and which do not destroy thesolid state of the powder coating material of the invention and of theslurry of the invention.

Examples of suitable oligomers and polymers d) are linear and/orbranched and/or block, comb and/or random poly(meth)acrylates,polyesters, polyurethanes, acrylated polyurethanes, acrylatedpolyesters, polylactones, polycarbonates, polyethers,(meth)acrylatediols, polyureas or oligomeric polyols, which function asreactive diluents for the thermal cure.

Where these oligomers and polymers are used as polyols d), theypreferably contain no carboxyl groups.

Examples of suitable reactive diluents d) for the thermal cure areoligomeric polyols which are obtainable from oligomericintermediates—themselves obtained by metathesis reactions of acyclicmonoolefins and cyclic monoolefins—by hydroformylation and subsequenthydrogenation; examples of suitable cyclic monoolefins are cyclobutene,cyclopentene, cyclohexene, cyclooctene, cycloheptene, norbonene or7-oxanorbonene; examples of suitable acyclic monoolefins are containedin hydrocarbon mixtures which are obtained in the processing ofpetroleum by cracking (C₅ cut); examples of suitable oligomeric polyolsfor use in accordance with the invention have a hydroxyl number of from200 to 450, a number-average molecular weight Mn of from 400 to 1000,and a mass-average molecular weight Mw of from 600 to 1100;

Further examples of suitable polyols d) are branched, cyclic and/oracyclic C₉-C₁₆ alkanes that are functionalized with at least twohydroxyl groups, especially diethyloctanediols.

Further examples of polyols d) for use are hyperbranched compoundscontaining a tetrafunctional central group, derived byditrimethylolpropane, diglycerol, ditrimethylolethane, pentaerythritol,tetrakis(2-hydroxyethyl)methane, tetrakis(3-hydroxypropyl)methane or2,2-bishydroxymethyl-1,4-butanediol (homopentaerythritol). Thesereactive diluents may be prepared in accordance with the customary andknown methods of preparing hyperbranched and dendrimer compounds.Suitable synthesis methods are described, for example, in the patents WO93/17060 or WO 96/12754 or in the book by G. R. Newkome, C. N.Moorefield and F. Vögtle, “Dendritic Molecules, Concepts, Syntheses,Perspectives”, VCH, Weinheim, N.Y., 1996.

These polyols d) are known to the skilled worker, and many suitablecompounds are available on the market.

The powder clearcoat slurry of the invention may, as component e),comprise constituents for curing with actinic radiation.

Suitable constituents e) include in principle all low molecular mass,oligomeric, and polymeric compounds that are curable with actinic lightand/or electron beams, and as are commonly used in the field ofUV-curable or electron beam-curable coating compositions. Theseradiation-curable coating compositions normally include at least one,preferably two or more, radiation-curable binders, based in particularon ethylenically unsaturated prepolymers and/or ethylenicallyunsaturated oligomers, where appropriate one or more reactive diluents,and where appropriate one or more photoinitiators.

It is advantageous to use the radiation-curable binders as constituentse). Examples of suitable radiation-curable binders e) are(meth)acryloyl-functional (meth)acrylic copolymers, polyether acrylates,polyester acrylates, unsaturated polyesters, epoxy acrylates, urethaneacrylates, amino acrylates, melamine acrylates, silicone acrylates, andthe corresponding methacrylates. It is preferred to use binders e) whichare free from aromatic structural units. Preference is therefore givento using urethane (meth)acrylates and/or polyester (meth)acrylates, withparticular preference aliphatic urethane acrylates.

Further examples of suitable constituents e) are radiation-curablereactive diluents such as low molecular mass polyfunctionalethylenically unsaturated compounds. Examples of suitable compounds ofthis kind are esters of acrylic acid with polyols, such as neopentylglycol diacrylate, trimethylolpropane triacrylate, pentaerythritoltriacrylate or pentaerythritol tetraacrylate; or reaction products ofhydroxyalkyl acrylates with polyisocyanates, especially aliphaticpolyisocyanates.

Examples of suitable photoinitiators e) are photoinitiators of theNorrish II type, whose mechanism of action is based on an intramolecularvariant of the hydrogen abstraction reactions such as occur diversely inphotochemical reactions, or cationic photo-initiators. By way ofexample, reference may be made here to Rompp Chemie Lexicon, 9thexpanded and revised edition, Georg Thieme Verlag Stuttgart, vol. 4,1991, or Rompp Lexikon Lacke und Druckfarben, Georg Thieme VerlagStuttgart, New York, 1998, pages 444 to 446.

If the powder clearcoat slurry of the invention is curable only withactinic radiation, it preferably contains only these constituents e).

The powder clearcoat slurry of the invention may further compriseeffective amounts of customary and known coatings additives f), such asare commonly used in clearcoat materials. Examples of suitable coatingsadditives f) are

UV absorbers;

free-radical scavengers;

crosslinking catalysts

slip additives;

polymerization inhibitors;

defoamers;

emulsifiers, especially nonionic emulsifiers such as alkoxylatedalkanols and polyols, phenols and alkyl phenols or anionic emulsifierssuch as alkali metal salts or ammonium salts of alkanecarboxylic acids,alkanesulfonic acids, and sulfo acids of alkoxylated alkanols andpolyols, phenols, and alkylphenols;

wetting agents such as siloxanes, fluorine compounds, carboxylicmonoesters, phosphoric esters, polyacrylic acids and their copolymers,or polurethanes;

adhesion promoters;

leveling agents;

film-forming auxiliaries such as cellulose derivatives;

flame retardants; or

rheologic control additives, such as those known from the patents WO94/22968, EP-A-0 276 501, EP-A-0 249 201 or WO 97/12945; crosslinkedpolymeric microparticles, such as are disclosed, for example, in EP-A-0008 127; inorganic phyllosilicates, such as aluminum magnesiumsilicates, sodium magnesium phyllosilicates and sodium magnesiumfluorine lithium phyllosilicates of the montmorillonite type; silicassuch as Aerosils; or synthetic polymers containing ionic and/orassociative groups such as polyvinyl alcohol, poly(meth)acrylamide,poly(meth)acrylic acid, polyvinylpyrrolidone, styrene-maleic anhydridecopolymers or ethylene-maleic anhydride copolymers and theirderivatives, or hydrophobically modified ethoxylated urethanes orpolyacrylates; or

Further examples of suitable coatings additives are described in thetextbook “Lackadditive” [Additives for coatings] by Johan Bieleman,Wiley-VCH, Weinheim, N.Y., 1998.

EXAMPLES AND COMPARATIVE EXPERIMENT Example 1

Preparation of the Acrylate Resin

21.1 parts of xylene were introduced into a vessel and heated to 130° C.Metered into this initial charge within 4 hours at 130° C., from twoseparate feed vessels, were the initiator, consisting of 4.5 parts ofTBPEH (tert-butyl perethylhexanoate) and 4.86 parts of xylene, and themonomer mixture, consisting of 10.78 parts of methyl methacrylate, 25.5parts of n-butyl methacrylate, 17.39 parts of styrene and 23.95 parts ofglycidyl methacrylate. The mixture was then heated to 180° C. and thesolvent was stripped off in vacuo at <100 mbar.

Example 2

Powder Coating Material for Subsequent Use in the Powder ClearcoatSlurry

73.5 parts of acrylate resin as per example 1, 17.8 parts ofdodecanedicarboxylic acid, 0.5 parts oftris(alkoxylcarbonylamino)triazine, 2 parts of Tinuvin 1130 (UVabsorber), 0.9 parts of Tinuvin 144 (HALS) and 0.4 parts of Additol XL490 (leveling agent) were intimately mixed in a Henschel fluid mixer,extruded on a BUSS PLK 46 extruder, ground on a Hosokawa ACM 2 mill, andscreened via a 125 um sieve.

Comparative Experiment CI

Preparation of a Powder Clearcoat Slurry

0.6 parts of Troykyd D777 (defoamer), 0.6 parts of Orotan 731 K(dispersing aid), 0.06 parts of Surfynol TMN 6 (wetting agent) and 16.5parts of RM8 (Rohm & Haas, nonionic associative thickener based onpolyurethane) were dispersed in 400 parts of deionized water. Then, insmall portions, 94 parts of powder coating material from example 2 werestirred in. Subsequently, a further 0.6 parts of Troykyd D777, 0.6 partsof Orotan 731 K, 0.06 parts of Surfynol TMN 6 and 16.5 parts of RM8 weredispersed in. Finally, in small portions, 94 parts of the powderclearcoat were stirred in. The material was ground in a sand mill for3.5 hours. The average particle size measured at the end is 4 μm. Thematerial was filtered through a 50 μm filter, and finally 0.05% of Byk345 (leveling agent) were added.

Example 3

Preparation of the Inventive Powder Clearcoat Slurry of the InventionUsing Phosphoric Acid

1000 ppm of phosphoric acid as a 10% strength aqueous solution wereadded to the powder clearcoat slurry from comparative experiment C1.

Example 4/Comparative Experiment C2

Application of the Powder Clearcoat Slurries

The powder clearcoat slurries as per example 3 (example 4) and of thecomparative experiment C1 (comparative experiment C2) were applied usinga cup-type gun to steel panels coated with commercially customaryelectrocoat material and white-pigmented aqueous basecoat material. Themetal panel was flashed off at room temperature for 5 minutes and at 60°C. for minutes. It was subsequently baked at a temperature of 145° C.for 30 minutes.

Example 5

Application of a Comparison Coating Material for Testing the Yellowing

As a comparison standard for the subsequent yellowing investigation,instead of the powder clearcoat slurries, a commercially customarytwo-component clearcoat material (“2K”; FF95-0111, from BASF CoatingsAG) was applied by means of an electrostatic spray gun to a steel panelidentically coated beforehand in accordance with example 4. This panelwas subsequently baked at a temperature of 140° C. for 30 minutes.

Example 6

Testing the Yellowing of Powder Clearcoat Slurries

From the samples prepared in accordance with example 4, as per example 4and comparative experiment C2, and from the comparison coating materialapplied in accordance with example 5, the yellow value (YV) wasdetermined in accordance with DIN 6127. The difference between theyellow value of the samples according to example 4 and comparativeexperiment C2 (Y_(sample)) and the yellow value of the comparisonstandard (2K; from example 5; YV₂K) was referred to subsequently as theyellow value difference (“delta-YV”) (delta-YV=YV_(sample)−YV_(2K)). Thehigher this value, the greater the yellowing and the difference from the2K clearcoat material, which from experience exhibits very littleyellowing.

TABLE 1 Delta-YV of the powder clearcoat slurries in accordance withexample 4 and comparative experiment C2 in comparison to the 2K coatingmaterial Sample delta-YV Comparative experiment C2 13.4 Example 4 1.9

As can be seen from table 1, the originally very high delta-YV of 13.4falls to 1.9 as a result of the effect of the phosphoric acid.

Example 7

Effect of the Addition of Phosphoric Acid to the Aqueous Base CoatMaterials 1 and 2

Example 4 was repeated except that in one case 10 ppm of phosphoric acid(example 7-1) and in one case 20 ppm of phosphoric acid (example 7-2),in each case in the form of a 10% strength aqueous solution, were addedto the aqueous basecoat material. This resulted in a further, albeitslight, lowering of the yellow value.

What is claimed is:
 1. A process for producing single-coat or multicoat clearcoat finishes on a primed or an unprimed substrate comprising applying at least one powder coating dispersion to the substrate to form at least one film, and then curing the at least one film by at least one of thermally and with actinic radiation, wherein the powder coating dispersion comprises a binder, a crosslinking agent, oxalic acid, and a polyvalent inorganic acid.
 2. The process as claimed in claim 1, wherein the polyvalent acid is selected from the group consisting of phosphoric acid, derivatives of phosphoric acid, sulfuric acid, derivatives of sulfuric acid, boric acid, and mixtures thereof.
 3. The process of claim 1, wherein the polyvalent inorganic acid comprises phosphoric acid.
 4. The process of claim 1, wherein the primed substrate is coated with at least one basecoat film.
 5. The process of claim 4, wherein the at least one basecoat film includes at least one of a polyvalent inorganic and oxalic acid.
 6. A powder coating dispersion comprising a binder, a crosslinking agent, oxalic acid, and a polyvalent inorganic acid.
 7. The powder coating dispersion of claim 6, wherein the polyvalent inorganic acid is selected from the group consisting of phosphoric acid, derivatives of phosphoric acid, sulfuric acid, derivatives of sulfuric acid, boric acid, and mixtures thereof.
 8. The powder coating dispersion of claim 6, wherein the polyvalent inorganic acid comprises phosphoric acid.
 9. A coated article comprising an article and a coating applied to the article, wherein the coating comprises the powder coating dispersion of claim 6 that has been cured.
 10. A coated substrate produced by the process of claim
 1. 11. The process of claim 1, wherein die polyvalent inorganic acid, except for phosphoric acid, is added to the powder coating dispersion in an amount from 0.1. to 2,000 ppm per entire weight of the powder coating dispersion.
 12. The powder coating dispersion of claim 6, wherein the polyvalent inorganic acid, except for phosphoric acid, is added to the powder coating dispersion in an amount from 0.1 to 2,000 ppm per entire weight of the powder coating dispersion. 